I have a check that was posted into my bank's checking account but it did not post in Quick Books, how can I fix this?

I appreciate you for letting us know about this here in the Community. I've got your back, and I'll ensure you can get the latest bank transactions to your QuickBooks Online (QBO) account.

You can manually update your bank account in QBO to refresh the connection between your bank and QuickBooks. This way, your transactions will flow properly into your QBO account. To begin, here's how:

1. Go to the Banking tab.
2. Select the bank account and click Update.
3. If prompted, enter your Multi-Factor Authentication (MFA) credentials and select Continue.

If you see the check in the For Review tab, you can click the Add button in the Action column to add  the transaction to your QuickBooks Online account. Here's how:

1. On the left navigational bar, go to the Banking tab.
2. In the For review section, locate the transaction.
3. Click Add below the Action column.

In case you still can't find the transaction, you consider manually recording it in your QBO account. See this article for further guidelines: Manually add transactions to account registers in QuickBooks Online.

Additionally, I've got you this article about categorizing and matching bank transactions in QBO: Categorize and match online bank transactions in QuickBooks Online.

You can also check this page to learn more about reconciling an account in QBO: Reconcile an account in QuickBooks Online.

Our door is open 24/7 if you need further assistance managing bank transactions in QuickBooks. You can also post here again if you have any additional QuickBooks-related concerns. It'll be my pleasure to help you out again. Take care, and have a good one.